No matter how exhilarating the romp, you still want to conk out as soon as you roll over. There are about as many theories for why this occurs as there are positions that get you to that state.

One of the most recent explanations comes from researchers in the Netherlands who study men’s brain activity in a PET scan before, during, and after orgasm.

What they’ve found: During orgasm, there’s a tremendous shift in blood flow throughout the brain. About 70 percent of the left side, which is involved in decision-making, is devoid of blood flow when you climax.

“With less blood flow, you can’t activate as many neurons and this can make you feel sleepy,” says Irwin Goldstein, M.D., director of sexual medicine at The Institute for Sexual Medicine in San Diego.

Chemically speaking, there’s also a tremendous release of endorphins right after ejaculation. “The endorphins are the natural equivalent of morphine,” says Jonathan Schiff, M.D., assistant clinical professor of urology at Mount Sinai School of Medicine in New York.

“They give you a very relaxed feeling, slow down your brain waves and cerebral functioning, and make you feel pleasantly tired.” Sweet dreams, indeed.

Photo: Stock4B/Corbis

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Your penis is sore.

As long as it’s a short-lived, sharp pain that only lasts a couple of seconds or minutes, then it’s likely normal and nothing to worry about.

You can release as fast as 25 miles per hour—not exactly a Maserati, but impressive nonetheless. If the pain lasts longer, like a few hours or a few days, you may have an infection in the testicles, epididymis, prostate, or urethra, or a sexually transmitted disease.

Visit your doc to rule out any serious issues, and most likely receive a prescription for an antibiotic.

“Otherwise the fluid would go back in through the bladder instead.” It can take some time for the muscle to relax after sex. “If you force it out, it may be a little painful—but it's harmless,” he adds. “It’s better to wait and let things relax before urinating.”

Photo: Ulf Huett Nilsson/Johner Images/Corbis

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You can't get hard again right away.

Welcome to the refractory period—the time after sex when it’s physiologically impossible for a man to climax again.

“After orgasm, the nerves have to regenerate their ability to sense stimulation in that area,” Dr. Schiff says.

It may also be due to the redistribution of blood flow through the brain that occurs during ejaculation, which needs to return to its baseline before you can come again. “Women, however, don’t have a refractory period and can experience multiple orgasms,” Dr. Goldstein says.

The length of your time-out depends on your age. In your early 20s it may be as little as 5 minutes. But enjoy it while you can, because as you get older it could take several hours, and for men over 50 it may take days.

Photo: Larry Washburn/fStop/Corbis

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Your package shrinks.

Just when you’re about to race over the edge of ecstasy, your testicles seem to disappear and you feel more like a show pony than the stallion you were just moments ago.

“When you ejaculate, the cremaster muscle contracts and brings your testicles up closer to your body, giving you the perception that they're smaller,” Dr. Schiff says. “This is the same muscle that causes shrinkage in cold water,” he adds.

During sex, there’s a lot of activity passed down to nerves in the spinal cord called S2, S3, and S4, which are involved in squeezing the muscles of the pelvis that help you ejaculate.

If you have a vigorous orgasm, the activity can cause sympathetic stimulation in a nearby nerve, S1, which plays a part in the sensation that goes to the outer part of the foot, especially the little toe.

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